Sleep complaints increase significantly with age in women. Many older women experience difficulty falling asleep, more night-time awakenings, and less restful sleep. Sleep studies verify that disturbed sleep patterns are observed even in healthy older women. Sleep disturbances are associated with increased daytime drowsiness, increased accident risk, increased use of health care, and reduced quality of life. Older women receive a disproportionate number of sedative-hypnotic medications, which can exacerbate sleep apnea and have daytime carryover effects such as sedation, falls and subsequent fractures, and cognitive impairment. A better understanding of the sleep changes experienced by older women is sorely needed. One contributing factor may be menopause-related changes in sex steroids such as estrogen and progesterone. Research attention has focused on estrogen. However, progesterone may also participate in the control of sleep. Clinical reports indicate that women often feel drowsy after they take oral progesterone - an effect which is undesirable during the day, but may be positive at night. To date there are no published studies of progesterone's effect on the objectively-measured sleep and daytime drowsiness of older women. The proposed study will take a systematic, multi-dimensional approach to determining the effect of progesterone on the sleep and drowsiness of older women. Objective techniques (polysomnography, Multiple Sleep Latency Test) will be used to measure sleep and daytime drowsiness following evening or morning administration of 300 mg micronized progesterone, in 40 postmenopausal women who are at least 5 years past menopause and who are not experiencing hot flashes. Attention, memory, subjective sleepiness, and blood levels of progesterone and its metabolite (allopregnanolone) will also be measured. This study is part of a long-term research plan to assess (1) how the very low postmenopausal levels of estrogen and progesterone contribute to sleep difficulties of older women, and (2) how hormone replacement therapy affects the sleep of women. An ongoing placebo-controlled study is investigating the effects of estrogen on the sleep of older women. The proposed study will complement the estrogen study. It will enhance our limited understanding of the relationship between sex steroids and sleep, and the factors that contribute to sleep problems in older women.